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Bridging the gap between operational and asset ratings – the UK experience and the green deal tool

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To truly understand how a building uses energy you need to know something about the building itself and about how it is used. In current parlance, that requires both an Asset and an Operational energy rating. An asset rating models the theoretical, as designed, energy efficiency of a particular building, based on the intrinsic performance potential of the building envelope (the fabric) and its services (such as heating, ventilation and lighting).The higher the numeric rating, the worse the building is, and the greater the opportunity to reduce carbon emissions by improving the building itself. However, the asset rating provides no information about how the building is operated in practice. The operational rating records the actual energy use from a building over the course of a year, and benchmarks it against buildings of similar quality, such as unregulated loads (e.g. IT, plug-in appliances) or building user behavior also create emissions, which are reflected in the operational rating. There is significant confusion in the non-domestic property market between the two different building energy ratings currently in use. Property owners mistrust asset ratings because they don’t map directly onto measured fuel bills. This paper discussed the underlying principles of how the UK Green Deal tool was designed, using the Interface to the Simple Building Energy Modeling (iSBEM), and how it has bridged the gap between the two ratings to provide a more complete picture of how energy is used in a building.
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Bridging the gap between operational and asset ratings
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KN5477 © BRE September 2013
Authors
Dr. A. J. Lewr y, CEng, MIMM, CEnv, SOE; Mr. J. Ortiz; EurIng, MASHRAE, MCIBSE; Dr. A. Nabil; Ms N. Schofield; Mr. R. Vaid; Mr. S. Hussain, and
Dr. P. Davidson, FInstP, FEI. Sustainable Energy Team, Building Research Establishment Ltd., UK. Lewrya@bre.co.uk
Summary
To truly understand how a building uses energy you need to know something about the building itself and about how it is used. In current
parlance, that requires both an Asset and an Operational energy rating. An asset rating models the theoretical, as designed, energy efficiency
of a particular building, based on the intrinsic performance potential of the building envelope (the fabric) and its ser vices (such as heating,
ventilation and lighting).The higher the numeric rating, the worse the building is, and the greater the oppor tunity to reduce carbon emissions by
improving the building itself. However, the asset rating provides no information about how the building is operated in practice.
The operational rating records the actual energy use from a building over the course of a year, and benchmarks it against buildings of similar
type. Factors other than building quality, such as unregulated loads (e.g. IT, plug-in appliances) or building user behavior also create emissions,
which are reflected in the operational rating.
There is significant confusion in the non-domestic property market between the two different building energy ratings currently in use. Property
owners mistrust asset ratings because they don’t map directly onto measured fuel bills.
This paper discussed the underlying principles of how the UK Green Deal tool was designed, using the Inter face to the Simple Building Energy
Modeling (iSBEM), and how it has bridged the gap between the two ratings to provide a more complete picture of how energy is used in a
building.
Keywords: Building Energy Modeling; Asset energy rating; Operational energy rating; Inter face to the Simple Building Energy Modeling
(iSBEM); Green Deal Tool.
Introduction
The rising cost of energy in the UK since 2000 has highlighted the
need for improved management of Energy. The UK’s Department of
Energy and Climate Change (DECC) updates its predictions of fossil
fuel prices annually (Department of Energy and Climate Change
(DECC), 2009). For example DECC’s modeling of gas prices based
on four scenarios - the worst of these scenarios predicts a 100%
increase in prices over the 10 years from 2008 (Lewry A. J., 2011).
As well as rising prices, security of energy supply has also become
an issue, particularly since the UK changed from being a net exporter
of gas to being a net importer in 2004. UK production satisfied
only about 70% of our demand in 2010 (Department of Trade and
Industr y (DTI), 2006). This loss of capacity has led to increasing
concern over energy security, as reported in an article in The
Guardian in January 2010 (Macalister T., 2010).
When managing energy one has to overcome the false perception
that it is a fixed cost to business and can be reduced only by tariff
negotiation. Considering energy as a variable cost to a business
provides the opportunity to discover the size of the potential savings.
Finally, there is legislation to comply with, examples of which are the
Climate Change Levy (CCL), Climate Change Agreements (CCAs)
with DECC and Industrial Emissions Directive (Directive 2010/75/EU,
2010), legislation and Environmental Permitting Regulations (The
Environmental Permitting (England and Wales) Regulations 2010)
which mainly cover industrial and manufacturing organisations.
These, along with the carbon reduction commitment (CRC)
energy efficiency scheme, are initiatives designed to help meet the
government ’s carbon reduction targets (Figure 1) to which energy
efficiency is a major contributor.
This all indicates the need for energy management as highlighted
in a recent review of best practice (Lewry A. J., 2012). The review
pointed out that establishing the facts and having a systematic
approach to data collection and analysis was essential for good
energy management. As part of this exercise both an asset and
operational rating were needed to truly understand how a building
uses energy.
Figure 1: UK government carbon reduction targets
(Sibback I., 2005)
There is significant confusion in the UK non-domestic property
market between the two different building energy ratings currently
in use. The legal requirement for the UK commercial sector is for
a calculated Energy Performance Certificate (EPC) which provides
an intrinsic Asset Rating. Public buildings have to display a Display
Energy Certificate (DEC), which is an Operational Rating based on
measured energy use. There is pressure to extend DECs into the
commercial sector - initially on a voluntary basis. The two ratings
measure different things - and each has its value. What is missing is a
means of relating one to the other.
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Bridging the gap between operational and asset ratings
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Asset and Operational ratings
The asset rating is a measure of building quality: the higher the
rating the worse the building is, and the greater the opportunity to
reduce carbon emissions and improve the building itself. However,
the asset rating provides no information about how the building
is operated in practice. The operational rating records the actual
energy use in a building over the course of a year, and benchmarks
it against buildings of similar type. An asset rating models the
theoretical, as-designed energy efficiency of a particular building,
based on the performance potential of the building itself (the
fabric) and its services (such as heating, ventilation and lighting).
Therefore, to understand and manage the energy use in a
building, both ratings are required as they show different aspects
of a building’s total energy performance.
The building quality (provided by the asset rating) has a large
impact on the total emissions, but does not explain all emissions.
Other factors such as unregulated loads (e.g. IT, plug-in
appliances) or building user behavior can also create emissions,
which are reflected in the operational rating.
Two offices with the same asset rating could have very different
operational ratings – a building with a low rating is used well by its
occupants, a building with a high rating is used badly. In the latter,
measures to change the behavior of the end users will be the best
option for reducing energy use and carbon emissions.
An example of an asset rating is an EPC, as produced for buildings
in the UK. One of the software tools used to create an EPC is
the interface to the Simplified Building Energy Model (iSBEM).
This was produced by BRE in 2006 for the UK Department for
Communities and Local Government (DCLG) in England and Wales
as a mechanism for calculating the energy used by buildings,
and forms part of the department’s process for implementing
the EU’s Energy Performance of Buildings Directive (Johnson T.,
2010). An EPC can be generated using the tool iSBEM, the free
downloadable user interface for SBEM (www. ncm.bre.co.uk)
that was also developed by BRE for DCLG.
The asset rating is intended to inform people on first occupancy,
i.e. at the point of construction, sale or rent, in order to help
purchasers or tenants in selecting the right building. At this point
in time, any previous metered information is not very helpful as the
previous occupants’ operation of the building, unregulated energy
use, etc. could be quite different to that of the new occupants.
An example of an operational rating is a DEC that is required
in the UK by all larger public buildings. The Operational
Rating Calculation (ORCalc) is the software used to calculate
the operational rating of a building from annual utility
consumption and to produce the DEC and an advisory report
(www.ukreg-accreditation.org).
BRE’s Building Energy Modeling team identified a possible solution
to the problem of linking the two rating methods, which has been
rolled out as an Audit tool - the Mauritian Building Energy Audit
Tool (MBEAT). This tool is able to join the two ratings together for
the purpose of an Energy Audit. This tool was a pre-cursor in that it
was simplified by the fact that only cooling needed to be considered
which was satisfied by a single fuel type – electricity.
Using the lessons learnt from the production of MBEAT (Lewry
A.J. et al, 2012), members of the team developed the Green Deal
assessment tool for non-domestic buildings for the Department for
Energy and Climate Change (DECC).
The Mauritian Building Energy Audit Tool (MBEAT)
The MBEAT tool comprises a calculation engine with a user interface.
The purpose of MBEAT and its interface is to produce consistent
and reliable evaluations of energy use in non-domestic buildings
for energy auditing purposes. MBEAT consists of a calculation
methodology (described in the sections below), which runs together
with an Energy Audit generator (EAgenMA) which utilises some of
the same data during the calculation. The user sees the interface
software, which interweaves these components together and
interacts with a series of databases to provide consistent data to the
calculation while simplifying the user’s need to obtain raw building
construction data.
Defining the Asset
When comparing the Asset and its operation performance one must
first define the building. There are a number of stages to inputting a
building in iSBEM:
a. Enter general information about the building, the owner, and
the energy auditor, and select the appropriate weather data.
b. Build up a database of the different forms of constructions and
glazing types used in the fabric of the building.
c. After “zoning” the building (on the drawings), create the zones
in the interface, and enter their basic dimensions, along with the
air permeability of the space.
d. Define the envelopes of each zone, i.e., walls, floor, ceiling,
etc. The envelopes’ areas, orientations, the conditions of the
adjacent spaces, and the constructions used all need to be
defined.
e. Within each envelope element, there may be windows/
rooflights or doors. The areas and types of glazing or door
within each envelope element need to be entered.
f. Define the HVAC (heating, ventilation, and air conditioning)
systems, the HWS (hot water systems), and any SES (solar
energy systems), PVS (photovoltaic systems), wind generators,
or CHP (combined heat and power) generators used in the
building.
g. Define the lighting system and local ventilation characteristics
of each zone, and assign the zones to the appropriate HVAC
system and HWS.
h. Run the calculation and assess energy performance.
The building services systems, zones, envelope elements, windows,
and doors are all referred to as “building objects” in iSBEM. Each of
these building objects is linked together so that iSBEM can calculate
the energy consumption of the building.
iSBEM calculates the energy demands of each space in the building
according to the activity within it. Different activities may have
different temperatures, operating periods, lighting standards,
etc. iSBEM calculates heating and cooling energy demands by
carrying out an energy balance based on monthly average weather
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conditions. This is combined with information about system
efficiencies in order to determine the energy consumption. The
energy used for lighting and hot water is also calculated. This
requires information from the following sources shown in Table 1:
Table 1: Calculation parameters for iSBEM
Information Source
Building geometry such
as areas, orientation,
etc.
Energy auditor reads from drawings or
direct measurement.
Weather data Internal database.
Selection of occupancy
profiles for activity
areas
For consistency, these come from an
internal Activity Database – energy
auditor selects by choosing building
type and activity from the database for
each zone.
Activity assigned to
each space
Energy auditor defines within MBEAT
by selecting from internal database
(the user should identify suitable zones
for the analysis by examining the
building or drawings).
Building envelope
constructions
Energy auditor selects from internal
Construction and Glazing databases
or inputs parameters directly. Energy
auditor can also define their own
constructions in the user-defined
construction database.
HVAC systems Energy auditor selects from internal
databases or inputs parameters
directly.
Lighting Energy auditor selects from internal
databases or inputs parameters
directly.
Defining a poorly managed asset
MBEAT is unique in that it compares and adjusts both the asset and
operation energy usage of a building. However, in order to adjust the
asset energy usage, one must first address the issue of the Poorly
Energy Managed Building (PEMB) definition.
The Poorly Energy Managed Building (PEMB) definition
The PEMB is needed to calculate one end of a scale between well
managed (equivalent to the asset energy usage, where the building
is perfectly controlled to the requirements of the activity databases)
and poorly managed (where the activity database parameters
are not adhered to). A separate scoring exercise places the actual
building on this scale, which is transposed from the calculated to
an “actual” scale. The position on the scale indicates where the
metered performance is expected to be, and hence the theoretical
split between asset and operational performance can be transposed
onto the actual scale, and theoretical predictions about the impact of
improvements can also be transposed to the actual scale.
How might the activity database parameters be
degraded?
If a zone is not controlled to the “ideal” set points and timings in
the database for the activity in that space, it can be regarded as
inadequately managed. Alternatively, some parameters might
change as a result of overloading rather than mismanagement. The
question is: how far might they be expected to drift before the zone
and building can be considered “poorly” managed? And in which
direction might they drift?
a. The amount of change that constitutes poor management, or
results from some issue, over which the energy manager has no
control, has to be a judgment based on what could reasonably
be expected in the situation
b. The direction we are concerned with is that which causes
energy consumption to rise.
Defining and quantifying energy management within
the asset
The definition of the PEMB allows the sliding scale to the well
managed building to be calibrated. However, the extent of the
energy management within the building needs now to be defined
and quantified so that it can be positioned on this scale.
As a starting point the authors looked at a tool developed for The
Energy Efficiency Best Practice programme, in the 90’s – “Energy
management priorities - a self-assessment tool” (UK Government’s
Energy Efficiency Best Practice Programme, 2001). This tool uses
Energy management matrices which are performance based and are
underpinned using detailed matrices covering all the technologies
within the built environment within the UK.
The weakness of these matrices is that they give an equal weighting to
each of the energy management issues and technologies considered.
In addition, there is a need to identify any new parameters which
are particular to the built environment of the climatic zone being
considered and filter out those which are not relevant.
With MBEAT the production of a series of new matrices with
weighted scores was initially carried out through a series of
information and data gathering exercises which engaged the
building professionals in Mauritius. This was then tailor by producing
an energy management tool which dovetailed to the asset tool.
The Energy Management tool
The energy management tool for MBEAT, which is contained
within a locked excel workbook, calculates an energy management
score. This score is calculated on the basis of the data collected by
the auditor from the real building. The tool contains a number of
worksheets which address all the energy management issues
Operational data
As well as the energy management score obtained from the
worksheet, meter data also needs to be entered. The metered data
needs to be of a full year so that any seasonal variations are ironed
out and each of the fuels types used within the building is entered
separately in the tool. Once the metered data has been entered so
can the energy management score based upon the audit
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With MBEAT this score is entered into the tool with the model in
Asset energy mode. The normalised management score of between
0 and 100 is entered in the current field in the management scores.
MBEAT calculations and outputs
The MBEAT tool calculates the asset rating using the standard
activities and weather file contained within the tool. Using the
metered data and initial energy management score entered by
the auditor it then calculates a Transposed Asset and adjusted
Operational performance using this data to correct for actual
patterns of usage and microclimate.
Once the Asset and Operational performance is calculated, MBEAT
then uses the improved energy management score to determine
the:
a. The potential operational saving for the building if management
improvements were applied based on the energy performance
of the building that corresponds to the current and improved
management scores as input by the energy auditor.
b. The potential asset saving if asset improvements were applied
to the building, based on the energy performance of the current
and improved building models as input by the energy auditor.
The mechanics of these calculations in MBEAT can be seen in
Figure 2.
The resulting MBEAT outputs are shown in Figure 3, where the
example building in question has all the potential savings as
operational, indicating this is where investment should be targeted.
However, at this stage it should be remembered that operational
savings relate to how the building is run and not the quality of the
asset.
Figure 3: Screen shot of MBEAT ratings Tab
MBEAT suitability and adaptability
The calculation procedure implemented in MBEAT is suitable for use
with the majority of buildings, but some designs will contain features
that mean that more accurate energy calculations may be obtained
by more sophisticated calculation methods.
All calculation processes involve some approximations and
compromises, and iSBEM is no exception. The most obvious
limitations relate to the use of the CEN monthly heat balance
method. This means that processes which vary non-linearly at
shorter time-steps have to be approximated or represented by
monthly parameters. The HVAC system efficiencies are an example
of this. On the other hand, iSBEM does have provision to account for
processes that may not be present in software packages that contain
more sophisticated fabric heat flow algorithms, such as duct leakage
and infiltration allowances.
MBEAT was designed to be used within the construction types,
practices, activities and climate of Mauritius with its scenario of a
single serving strategy and fuel type.
In addition, the important Energy Management Issues need to be
captured in order to populate the matrices. Alongside this each
issue needs to be ranked and weighted to that a quantifiable Energy
Management score can be produced.
Despite its limitations, MBEAT provided a fledgling methodology
for linking the Asset and Operational performance of a building
and thus bridging the gap between the two measures. The result
was a more holistic view of building performance and a tool that
allows possible savings to be quantified with more confidence and
improvements to be prioritised.
The approach described will help to understand and improve the
comparison between existing (asset and/or operational) approaches.
Evidence from the initial use of the MBEAT tool in Mauritius,
indicated that the underpinning methodology allows the asset
and operational performance of a building to be compared for the
purpose of highlighting where investment should be targeted.
Figure 2: Details of the MBEAT calculations
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The Green Deal assessment tool
The MBEAT tool had proved a valuable learning exercise but for
this to be imported back into the UK following barriers need to be
overcome:
a. How to capture the energy management scores within the
iSBEM tool.
b. How to deal with multiple servicing strategies and several fuel
types.
This has led to the Green Deal assessment tool abandoning the
ideal of a single energy management score for the whole building
but instead having management scores for each individual building
object and integrating these within the tool. This provides granularity
for the fabric and services, as well as taking into account different
fuel types because the fuel type is assigned at this level. As a result
the transposition is done at an object level and then re-aggregated
to give the overall level of performance of the building. This has led
to a tool with a more in-depth picture of energy usage within the
building where asset and operational rating can be used to quantify
and prioritise investment (see Figure 4).
References
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on DECC fossil fuel price assumptions, 2009, Available at:
www.bis.gov.uk/files/file51365.pdf
Lewry A J., Literature review on the use of forecasted energy
and water prices in construction projects. SCI-Network
Report 4.2. London, SCI-Network, 2011, Available at:
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Department of Trade and Industry (DTI), JESS: long-term security of
energy supply, London, DTI, 2006.
Macalister T., Energy security questioned as National Grid cuts
off gas to factories, The Guardian, 7 January 2010, Available at:
www.guardian.co.uk.
Directive 2010/75/EU, Directive on industrial emissions, Brussels,
European Parliament and Council of the European Union, 24
November 2010.
The Environmental Permitting (England and Wales) Regulations
2010. SI 675, London, TSO, 2010.
Lewry A J., Energy Management in the Built Environment – A review
of best practice, BRE FB 44. Bracknell, IHS BRE Press, 2012.
Sibback I., Carbon Trust: innovation funding. Presentation given
at ‘Sustainable Renewable Energy Sources: an Environmental
Challenge’ meeting, 21 March 2005, Manchester Metropolitan
University.
Johnson T., SBEM for non-domestic buildings: an introduction. BRE IP
7/10. Bracknell, IHS BRE Press, 2010.
Lewry A. J., Or tiz J., Nabil, A., Johnson A ., Hussain S., and Davidson
P., “Bridging the gap” - a tool for energy auditing that encompasses
both asset and operational parameters, CIBSE ASHRAE Technical
Symposium, Imperial College, London UK – 18th and 19th April
2012.
UK Government’s Energy Efficiency Best Practice Programme, Energy
management priorities - a self-assessment tool (GPG306), 2001.Figure 4: Green Deal Tool overall process
... The higher the rating the worse the building is, and the greater the opportunity to reduce carbon emissions and improve the building itself. However, the asset rating provides no information about how the building is operated in practice [10]. ...
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SBEM for non-domestic buildings: an introduction
  • T Johnson
Johnson T., SBEM for non-domestic buildings: an introduction. BRE IP 7/10. Bracknell, IHS BRE Press, 2010.
Carbon Trust: innovation funding. Presentation given at 'Sustainable Renewable Energy Sources: an Environmental Challenge' meeting
  • I Sibback
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